The present invention relates to a fishfinder for identifying a single fish and for measuring the length of a fish.
In general, there have been used in fishing vessels fishfinders displaying echo signals in colors depending on the signal strength of received echo signals on the two-dimensional screen of an indicator having two mutually perpendicular axes with one axis representing an elapsed time or a vessel travel distance and with the other axis representing water depth. An operator of a fishfinder looks at echo images displayed two-dimensionally on the screen of an indicator to grasp the signal strength of received echo signals respectively in relation to water depths thereof.
Japanese Laid-Open Patent Publication No. H04-339288 discloses a fishfinder for displaying fish-like mark representing a fish school with its mark size depending on the signal strength of a fish echo signal, as superposed on echo images. U.S. Pat. No. 6,628,569B1 discloses a fishfinder for displaying a fish-like mark and for indicating a numerical value representative of the water depth of a fish school in the proximity of an echo image thereof. When these displays presented by the fishfinders disclosed in the patent publications, an operator can instantly grasp the water depth at which a fish school is. However, these display methods could not display signal strengths of received signals precisely. The signal strength is determined based on Target Strength which is peculiar to each target and numbers of the targets. The length of a single fish can be estimated based on Target Strength thereof. Japanese Patent No. 2,883,679 discloses a fishfinder for measuring more precisely the target strength of a single fish. The fishfinder is comprised with a transmission and reception device having a narrow angle radiation characteristics and another transmission and reception device having a radiation characteristics a portion of which has a substantially equal sensitivity characteristics. There is used echo signals resulting from an ultrasound pulse transmitted in a narrow angle beam pattern to judge whether a target directly under the hull of a ship is a single fish or not. With a single fish determined, there is used an echo signal resulting from an ultrasound pulse emitted by the transmission and reception device having a radiation characteristics having a substantially equal sensitivity characteristics, which is in synchronism with the single fish echo signal resulting from an ultrasound pulse emitted by the transmission and reception device having a narrow angle radiation characteristics. The transmission and reception device having a substantially equal sensitivity characteristics in the radiation characteristics is complicated in construction and large in size. There is used only an ultrasound pulse transmitted from the transmission and reception device having a narrow angle radiation characteristics to distinguish a single fish. It will not be possible to determine whether a detected fish is directly under the hull of a ship or is horizontally away from a vertical imaginary plane passing through a straight line connecting the bow and the stern of the ship. There have been misinterpretations in determining a single fish of a school of fish. The fishfinders described in these patent documents obtain target strength of a single fish based on echo signals received by a special ultrasound transducer having a substantially equal sensitivity characteristics in the radiation characteristics requiring no error corrections. However, with the fishfinders described in the above patent publications, it has been necessary for an operator to distinguish a single fish and to estimate the length of a fish which requires sophisticated knowledge and many experiences. The applicant filed a Japanese patent application about a fishfinder for identifying a single fish and measuring the length of a fish (refer to Japanese Patent Laid-Open No. 2005-249398). With this fishfinder, it collects statistics about a plurality of fishes in an area selected by an operator, and a histogram in terms of fish lengths is drawn. The display has enabled an operator to grasp fish length distribution of single fishes in the area selected.
In distinguishing a single fish or in measuring target strength or in measuring the length of a single fish, prior art fishfinders have mistaken, in some cases, a fish school for a single fish and vice versa. In such cases, there has not been measured with high precision target strength of a single fish or the length of a single fish. Further, in order to measure target strength of a single fish or the length of a fish without having sensitivity adjustments made, it has been necessary to use a special transmission and reception device complicatedly constructed and being large in size and having an equal sensitivity directional pattern over an azimuthal range. The conventional display method displays fish length distribution of single fishes in each area. It has been difficult to grasp a fish location and a fish length instantly.